Thin and thick film resistors are commonly used for sensors and electronics resistors in hybrid, integrated and printed circuit board-level electrical circuits. Metal resistors may include platinum resistance thermometers for temperature sensing, nickel/chrome alloys for low temperature coefficient of resistance (TCR) electronics resistors, and the like. Metal films have inherently low resistivities (specific resistances) and thus must be made in long "wire" shapes, usually folded into a serpentine configuration, in order to provide a resistance of sufficiently large magnitude so that a useable voltage drop (signal) can be obtained.
Conventional metal film resistors having serpentine configurations, however, occupy a considerable amount of valuable circuit area which, in turn, limits the range of resistance values available to the circuit designer. Nonetheless, the resistance values of such resistors can be adjusted (i.e., trimmed) easily to accurate values by providing "loops" or "links" of excess conducting material that can be cut by various methods (for example, lasers, sandblasters, ultrasonic cutting tools, miniature saws, or the like) without disturbing the remaining conducting material.
In the case of higher resistivity materials, such as ruthenium dioxide or bismuth ruthenate thick film materials, the high resistivity allows adequate resistance values to be obtained in a simple rectangle or square shape with electrical contacts on either side. For these resistors, trimming is usually accomplished by cutting into the sides of the active bodies of the resistors with a laser, although occasionally a few trimmable "links" are incorporated physically into the resistors. Highly accurate resistor-to-resistor uniformities can thus be obtained by real time measurement of the resistance value while the resistor is being trimmed. For real time measurements on resistor materials having a high temperature coefficient of resistance (TCR) during laser trimming, heat sinking or some other compensating method must be practiced in order to obtain the highest accuracy.
The usefulness of the laser trimming method is greatly enhanced, however, when employed to trim electronic circuit film resistors having very low TCR's because the laser heat has a minimal effect on the measured resistance value. However, even with such low TCR resistors, the laser trimming method can melt bordering material during the trimming operation which changes the nature of that material thereby usually causing drift in the resistance values. As a result, the trimming accuracy is decreased and the overall cost of the resistor is increased.
Thermistors having either a negative temperature coefficient of resistance (NTCR) whereby resistance decreases with an increase in temperature, or high positive temperature coefficient of resistance (PTCR) whereby resistance increases with an increase in temperature, are special applications of high resistivity resistors because they are used to measure temperature, control current surges or to prevent thermal run-away in electronic circuits. A relatively large resistivity change per unit temperature is desirable in such thermistors to allow a sufficiently large signal to be generated. However, a large resistivity change limits the useful temperature range of the thermistor since the resistance value will quickly increase to an extent whereby excessive errors in the measuring circuit result. Thus, if a thin or thick film with, for example, a high resistivity and high NTCR is to be used, a simple square or rectangular geometry of the active resistance material is not adequate.
In such situations, the overall resistance of conventional thermistor devices is frequently decreased (without decreasing the material-specific resistivity and temperature coefficient) by reducing the distance between the pair of electrodes and increasing the body width of the resistor. For example, as disclosed in U.S. Pat. No. 4,359,372, a reduced distance between electrodes and increased body width of the resistor can be embodied in a serpentine configuration so that the sum of the resistor body and electrical contacts can in effect mimic a more "rectangular" shape. However, there is a practical limit to this conventional technique since ultra-fine electrode geometries typically exceed the capabilities of thick film fabrication technology. Furthermore, accurate trimming of conventional devices of the type described in U.S. Pat. No. 4,359,372 (see FIGS. 3-5 therein) becomes increasingly more difficult to accomplish as the electrode geometry becomes more fine and/or complex.
What has been needed in this art, therefore, is a film resistor having an electrode geometry which can more easily and accurately be trimmed to a desired resistance value without disturbing the active body of the resistor (e.g., upsetting the temperature equilibrium of the resistor during trimming or causing drift by changing the material of the body). It is towards fulfilling such a need that the present invention is directed.
Broadly, the present invention is embodied in a film resistor having an opposed pair of electrodes laterally positioned with respect to an active resistor body so as to establish a region of the insulating substrate interposed between each electrode and the active resistor body. The electrodes, moreover, are most preferably provided with interdigitated fingers which extend across (i.e., bridge) a respective one of the substrate regions and into electrical communication (contact) with the active resistor body.
Thus, according to the present invention, the resistance value of a high resistivity film resistor device can, for example, be adjusted in a gross manner so as to lower drastically the resistance value to within values useable by conventional circuitry (e.g., by factors of 100), while also permitting selective and precise (fine) resistance value adjustment (e.g., by trimming to about 1% of the desired resistance value). Furthermore, resistance trimming can be accomplished with the film resistors of this invention without compromising the active resistor body simply by severing one or more of the fingers from the remaining electrode material at a location which is coextensive with the substrate region. The temperature of the resistors of this invention also does not necessarily need to be controlled during the trimming operation (but could be, if desired) since the finger(s) to be severed so as to achieve a desired resistance value can simply be computed once the overall resistance value at a given temperature is known.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments.